The present invention relates to an improved method for efficiently controlling the thickness of wafer-like work pieces under lapping and a lapping machine provided with such an improved means for in-machine controlling of the thickness of the wafer-like work pieces under lapping therein.
As is well known, a variety of wafer-like materials are currently used in the fields of, for example, electronics industry such as high-purity silicon semiconductors, GGG (gadolinium gallium garnet) single crystals for magnetic bubble domain memories, fused quartz glass plates used for masking in the patterning of IC circuits and the like. These materials are prepared first by slicing a block or rod of the respective materials into a wafer-like form which is then ground and lapped to have a specified thickness and surface smoothness in a lapping machine, for example, disclosed in U.S. Pat. No. 3,089,292.
Along with the development of the electronics technology, it is a recent trend that the requirement for the accuracy or uniformity of the thickness of such wafer-like materials as finished by lapping is increasing more and more. The most reliable way for the determination of the thickness of a wafer-like work piece under lapping is of course the direct measurement of the thickness of the work piece periodically taken out of the lapping machine by interrupting the operation. Needless to say, however, such an out-of-machine measurement of the thickness is impracticable with very low working efficiency.
Accordingly, there have been proposed several methods for the in-machine measurement of the thickness of the work pieces under lapping, i.e. without taking the work pieces out of the lapping machine to discontinue the operation of the lapping machine at a moment when the decreasing thickness of the work pieces under lapping has reached the desired or specified thickness.
One of the principles of such an in-machine thickness measurement is the utilization of an eddy current sensor fixedly mounted, for example, on the upper surface plate of the lapping machine to generate electric signals corresponding to the width of the gap between the upper and lower surface plates sandwiching the work pieces. The value obtained in such an eddy current sensor indirectly represents the thickness of the work pieces under lapping as the first approximation although several factors of error are involved therein.
For example, the value represented by the eddy current signal is not the exact thickness of the work piece but the width of the gap between the surface plates sandwiching the work pieces which is the summation of the thickness of the work piece and the thickness of the layers of the abrasive material or powder intervening between the surfaces of the work piece and the surfaces of the upper and lower surface plates.
Further, the surface of the surface plate itself is subject to abrasive wearing off throughout the course of lapping and the amount of wearing off of the upper surface plate holding the sensor gives a negative error to the value of the thickness of the work piece as represented by the eddy current signal.
Therefore, corrections should be made always on the value of the thickness of the work piece under lapping as determined from the signal obtained in the sensor mounted on the upper surface plate by subtracting the thickness of the layers of the abrasive material and by adding the thickness corresponding to the amount of wearing off of the upper surface plate estimated empirically.
Notwithstanding the above mentioned corrections effected on the value of the thickness of the work piece directly determined from the signal of the sensor means, the true thickness of the work piece taken out of the lapping machine after completion of the lapping often deviates considerably from the value expected from the recording of the in-machine measurement by use of the sensor means.
The inventors have conducted extensive investigations for the elucidation of the reason for the above mentioned deviation in the recording of the in-machine thickness measurement from the true thickness of the wafer-like work piece taken out of the lapping machine to establish an efficient and reliable method for controlling the thickness of the wafer-like work pieces under lapping.